Utilisation of oxygen evolving anode for Hall-Hèroult cells and design thereof

ABSTRACT

A method for electrolytic production of aluminium metal from an electrolytic ( 3 ) including aluminium oxide, by performing electrolysis, with at least one inert anode ( 1 ) and at least one cathode ( 2 ) thus forming part of an electorwinning cell. The anode evolves oxygen gas and the cathode has aluminium discharged onto it in the electrolysis process, where the oxygen gas enforces an electrolyte flow pattern. The oxygen gas is directed to flow into anode grooves and is drained away from the interpolar room, thereby establishing an electrolyte flow pattern between the electrodes ( 1 ) and ( 2 ) and between over the anodes ( 1 ). The invention also concerns and anode assembly and an electrowinning cell.

The present invention relates to a method for production of aluminium bythe use of at least one inert anode and the corresponding design of theanode and cell.

BACKGROUND ART

Aluminium is presently produced by electrolysis of analuminium-containing compound dissolved in a molten electrolyte, and theelectrowinning process is performed in cells of conventionalHall-Hèroult design. These electrolysis cells are equipped withhorizontally aligned electrodes, where the electrically conductiveanodes and cathodes of today's cells are made from carbon materials. Theelectrolyte is based on a mixture of sodium fluoride and aluminiumfluoride, with smaller additions of alkaline and alkaline earthfluorides. The electrowinning process takes place as the current passedthrough the electrolyte from the anode to the cathode causes theelectrical discharge of aluminium-containing ions at the cathode,producing molten aluminium, and the formation of carbon dioxide at theanode (see Haupin and Kvande, 2000). The overall reaction of the processcan be illustrated by the equation:2Al₂O₃+3C=4Al+3CO₂  (1)

Due to the horizontal electrode configuration, the preferred electrolytecomposition and the use of consumable carbon anodes, the currently usedHall-Hèroult process displays several shortcomings and weaknesses. Theseweaknesses include area-intensive design, high investment costs,troublesome electrolyte and metal flow patterns, expensive electricbusbar systems, etc.

The traditional aluminium production cells utilise carbon materials asthe electrically conductive cathode. Since carbon is not wetted bymolten aluminium, it is necessary to maintain a deep pool of moltenaluminium metal above the carbon cathode, and it is in fact the surfaceof the aluminium pool that is the “true” cathode in the present cells. Amajor drawback of this metal pool is that the high amperage of moderncells (>150 kA) creates considerable magnetic forces, disturbing. As aresult, the metal tends to move around in the cell causing wavemovements that might locally shortcut the cell and promote dissolutionof the produced aluminium into the electrolyte. In order to overcomethis problem, complex busbar systems are designed to compensate for themagnetic forces and to keep the metal pool as stable and flat aspossible. The complex busbar system is costly, and if the disturbance ofthe metal pool is too large, aluminium dissolution in the electrolytewill be enhanced, resulting in reduced current efficiency due to theback reaction:2Al+3CO₂=Al₂O₃+3CO  (2)

The preferred carbon anodes of today's cells are consumed in the processaccording to reaction (1), with a typical gross anode consumption of 500to 550 kg of carbon per tonne of aluminium produced. The use of carbonanodes results in the production of pollutant greenhouse gases like CO₂and CO in addition to the so-called PFC gases (CF₄, C₂F₆, etc.) whichare even more pollutant greenhouse gases and very stable. Theconsumption of the anode in the process means that the interpolardistance in the cell will constantly change, and the position of theanodes must be frequently adjusted to keep the optimum operatinginterpolar distance. Additionally, each anode is replaced with a newanode at regular intervals. Even though the carbon material and themanufacture of the anodes are relatively inexpensive, the handling ofthe used anodes (butts) makes up a major portion of the operating costin a modern primary aluminium smelter.

The raw material used in the Hall-Hèroult cells is aluminium oxide, alsocalled alumina. Alumina has a relatively low solubility in mostelectrolytes. In order to achieve sufficient alumina solubility, thetemperature of the molten electrolyte in the electrowinning cell must bekept high. Today, normal operating temperatures for Hall-Hèroult cellsare in the range 940-970° C. To maintain the high operatingtemperatures, a considerable amount of heat must be generated in thecell, and the major portion of the heat generation takes place in theinterpolar space between the electrodes. Due to the high electrolytetemperature, the side walls of today's aluminium production cells arenot resistant to the combination of oxidising gases and cryolite-basedmelts, so the cell side linings must be protected during cell operation.This is normally achieved by the formation of a crust of frozen bathledge on the side walls. The maintenance of this ledge necessitatesoperating conditions where high heat losses through the side walls is acardinal requirement. This results in the electrolytic production havingan energy consumption that is substantially higher than the theoreticalminimum for aluminium production. The high resistance of the bath in theinterpolar space accounts for 35-45% of the voltage losses in the cell.The state-of-the-art of present technology is cells operating at currentload sin the range 250-350 kA, with energy consumption around 13 kWh/kgAl and a current efficiency of 94-95%.

As pointed out, there are several good reasons for improving the celldesign and the electrode materials in aluminium electrolysis cells, andseveral attempts have been made to obtain these improvements.

With an inert anode in the electrowinning of aluminium, the overallreaction would be:2Al₂O₃=2Al+3O₂  (3)

Many attempts have been made to find the optimum inert anode materialand the Introduction of these materials in electrolytic cells, andnumerous patents have been proposed for inert anode materials foraluminium electrowinning. Most of the proposed inert anode materialshave been based on tin oxide and nickel ferrites, where the anodes maybe a pure oxide material or a cermet type material. The first work oninert anodes was initiated by C. M. Hall, who worked with copper metal(Cu) as a possible anode material in his electrolysis cells. Generally,the inert anodes can be divided into metal anodes, oxide-based ceramicanodes and cermets based on a combination of metals and oxide ceramics.The proposed oxide-containing inert anodes may be based on one or moremetal oxides, wherein the oxides may have different functions, as forinstance chemical “inertness” towards cryolite-based melts and highelectrical conductivity. The proposed differential behaviour of theoxides in the harsh environment of the electrolysis cell is, however,questionable. The metal phase in the cermet anodes may likewise be asingle metal or a combination of several metals (metal alloys). The mainproblem with all of the suggested anode materials is their chemicalresistance to the highly corrosive environment due to the evolution ofpure oxygen gas (1 bar) and the cryolite-based electrolyte. To reducethe problems of anode dissolution into the electrolyte, additions ofanode material components (U.S. Pat. No. 4,504,369) and a selfgenerating/repairing mixture of cerium based oxyfluoride compounds (U.S.Pat. Nos. 4,614,569, 4,680,049 and 4,683,037) have been suggested aspossible inhibitors of the electrochemical corrosion of the inertanodes. However, none of these systems have been demonstrated as viablesolutions.

The introduction of inert anodes and wettable cathodes in the presentHall-Hèroult electrowinning cells would have a significant impact onreducing the production of greenhouse gases like CO₂, CO and PFC's fromaluminium production. Also, potentially the reduction in energy addedcan be substantial if the inter-electrode space can be reduced incomparison to traditional Hall-Hèroult cells.

Patents regarding retrofit or enhanced development of Hall-Hèroult cellsare amongst others described in U.S. Pat. Nos. 4,504,366, 4,596,637,4,614,569, 4,737,247, 5,019,225, 5,279,715, 5,286,359 and 5,415,742, aswell as GB 2 076 021. All of these patents address the problemsencountered due to the high heat losses in the present Hall-Hèroultcells, and the electrolysis process is operated at reduced interpolardistances. Some of the proposed designs are in addition effective withrespect to reducing the surface area of the liquid aluminium metal padexposed to the electrolyte. However, only a few of the suggested designshave addressed the low production to area ratio of the Hall-Hèroultcells. Amongst others, U.S. Pat. Nos. 4,504,366, 5,279,715 and 5,415,742have tried to solve this problem by implementation of vertical electrodeconfigurations to increase the total electrode area of the cell. Thesethree patents have also suggested the use of bipolar electrodes. Themajor problem of the cell design suggested in these patents, however, isthat the requirement for a large aluminium pool on the cell bottom toprovide electrical contact for the cathodes. This will render the cellsusceptible to the influence of the magnetic fields created by thebusbar system, and may hence cause local short-circuiting of theelectrodes.

Additionally, the referred patents, as well as U.S. Pat. No. 6,030,518,all point to the lowering of the bath temperature as compared to normalHall-Hèroult cell temperatures as a means of a feasible reduction of theanode corrosion rates in the cell. The utilisation of the gas-lifteffect and design of so-called up-corner and down-corner flow funnelsare also described in U.S. Pat. No. 4,308,116, specially aimed atmagnesium production.

U.S. Pat. No. 4,681,671 describes a novel cell design with a horizontalcathode and several, blade-shaped vertical anodes, and the cell is thenoperated at low electrolyte temperatures and with an anodic currentdensity at or below a critical threshold value at which oxide-containinganions are discharged preferentially to fluoride anions. By means offorced or natural convection, the melt is circulated to a separatechamber or a separate unit, in which alumina is added before the melt iscirculated back into the electrolysis compartment. Although the totalsurface area of the anode is high in the proposed configuration, theeffective anode area is small and limited due to the low electricalconductivity of the anode material relative to the electrolyte. Thiswill substantially limit the useful anodic surface area, and will leadto high corrosion rates at the effective anode surface.

A fact well established in hydrodynamics is that the flow of a fluidsystem is governed by a balance between the driving force for fluid flowand the resistance to fluid flow within the components of the system.Furthermore, depending upon the configuration, the velocity within localregions flow may be in the same direction but may sometimes be in thedirection opposite to the fluid drive. This principle is amongst otherscited in U.S. Pat. Nos. 3,755,099, 4,151,061 and 4,308,116. Inclinedelectrode surfaces are used to enhance/facilitate the drainage of gasbubbles from the anode and molten metal from the cathode. Hence, thedesign of electrolysis cells with vertical or near horizontal electrodesof both multi-monopolar and bipolar electrode arrangement, where fixedinterpolar distance and the gas-lift effect are used to create a forcedconvection of the electrolyte flow, is not new. WO 02/31225 and U.S.Pat. Nos. 3,666,654, 3,779,699, 4,151,061 and 4,308,116, amongst othersutilise such design principles, and the two latter patents also givedescriptions of the use of “funnels” for up-comer(s) and down-comer(s)with respect to the electrolyte flow. U.S. Pat. No. 4,308,116 alsosuggests the use of a separation wall for enhanced separation ofproduced metal and gas. However, the inclined rod-shaped anodesdescribed in WO 02/31225 do not set up Such a strong and controlledbubble driven flow as the present invention, and experiments show thatgas will escape from all sides of such an anodes even if the bottomsurface is inclined several degrees.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method and anelectrowinning cell for production of aluminium by the electrowinning ofaluminous ore, preferably aluminium oxide, in a molten fluorideelectrolyte, preferably based on cryolite, at temperatures in the rangeof 680-980° C. The method is designed to operate at equal or lower costcompared to the present production technology for electrowinning ofaluminium, and thus provides a commercial and economically viableprocess for the production. This means the design of an electrolysiscell with the necessary cell components and outline to reduce energyconsumption, reduce overall production costs and still maintain highcurrent efficiency. The compact cell design is obtained by the use ofdimensionally stable anodes and aluminium wettable or non-wettablecathodes. The internal electrolyte flux is designed to attain a highdissolution rate of alumina, even at low electrolyte temperatures, and agood separation of the two products from the electrolysis process.Problems identified with the mentioned patents (U.S. Pat. Nos.4,681,671, 5,006,209, 5,725,744 and 5,938,914 and WO 02/31225) are alsonot encountered in this invention due to the more sophisticated designof the electrolysis cell.

Other Publications:

Haupin, W. and Kvande, H.: “Thermodynamics of electrochemical reductionof alumina”, Light Metals 2000, pp. 379-384.

Shekar, R. and Evans, J. W.: “Modeling studies of electrolyte flow andbubble behavior in advanced Hall cells”, Light Metals 1990, pp. 243-248.

Shekar, R. and Evans, J. W.: “Physical modeling of bubble phenomena,electrolyte flow and mass transfer in simulated advanced Hall cells.Final Report”, DOE/ID-10281, University of California, Berkeley, March1990.

Solheim. A. and Thonstad, J.: “Model cell studies of gas inducedresistance in Hall-Hèroult cells”, Light Metals 1986, pp. 397-403.

A governing principle in the present invention related to anelectrolysis cell for the accomplishment of aluminium electrolysis, andfor the construction principle of the aluminium electrowinning cell, isthat the two products, aluminium and oxygen, shall be efficientlycollected with minimal losses due to the recombination of theseproducts. This is sought realised through a cell design where anefficient and fast drainage of the produced gas from the inter polarroom in such a manner that the oxygen retention time, and therefor theback reaction between the products, are reduced to a minimum.

Oxygen bubbles are small compared to CO₂ which give significantly higherbubble layer resistance under oxygen generating horizontally orientedanodes compared to similar CO₂— generating anodes. This behaviour reducethe horizontal surface area the inert anode can have to achieve uniformcurrent distribution and low bubble layer resistance. The presentinvention takes care of the said limitation by reducing the length theproduced gas has to travel at the active anode surface combined with anefficient gas drainage.

The present design concept can be used to built a completely newpotline, but more importantly, the anode assembly can replace carbonanodes in most of the existing Hall-Heroult Prebake and Søderberg cellsproducing oxygen instead of CO₂ at the anode. The implementation and useof such retrofitted cells has a huge economical potential because theexisting potroom, cathode potlining, busbar systems, anode beam andinfrastructure can be used with a minimum of adjustments/changes. Oneway to retrofit a prebake cell by replacing carbon anodes underoperation has been described in WO Pat. Nos. 01/63012 A2, but the anodesdescribed here are very different from the present invention.

These and other advantages can be achieved by the invention as describedbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention shall be further described by figuresand an example where:

FIG. 1: Shows a schematic view of the horizontal cross sectiontransverse of an electrolysis cell according to the invention.

FIG. 2: Shows a horizontal cross section of the anode shown in FIG. 1.

FIG. 3: Shows a horizontal section of two anodes and the circulationpattern obtained by the shape of the grooves and the exterior surface ofthe invented anodes which are turned 90° compared to the ones in FIG. 1.

FIG. 4: Shows two examples of the bottom surface of anode clusters intwo cells facing s molten aluminium cathode with different orientationof the grooves.

FIG. 5: Shows an alternative anode shape where the bottom of the anodefacing the cathode can be shaped like a cone or a roof with 3 or moreplanes with angled or straight surfaces towards a hole in the top whereproduced anode gas can escape.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

FIGS. 1-3 disclose a cell for the electrowinning of aluminium comprisinginert anodes (1) immersed in electrolyte (3) and an aluminium poolserving as a cathode (2). In operation, the produced oxygen gas (10) atthe inert anode (1). The oxygen gas is produced at the inert anodeelectroactive surface (15), hereafter named the anode “tooth”. Theoxygene bubbles produced at the surface will follow the shape of thesideways sloped bottom of the anode (FIG. 2) into a groove (4). Thegrooves (4) have to be sloped 1-5° according to the horizontal metalsurface to efficiently and quickly transport the produced oxygen awayfrom the inter polar room (5) with a minimum of agitation and mixing ofproduced oxygen (10) and aluminium (2).

The end of the sloped grooves (4) should be rounded upwards at the ends(FIG. 3) to give smooth gas release and not a frequently pumping gasrelease. Grooved anodes have been proposed previously, but not saidangled grooves in horizontally oriented anodes (FIG. 3) where shapedanode “teeth” (15) according to the present invention are as much as10-20 cm wide.

The centre line at the bottom of the anode “tooth” (1) shown in FIGS. 3and 4. are parallel to the cathode surface, but there should be slopedsideways at the tooth angled 1-5° perpendicular to the centre linetowards the grooves (4). The surface of the anode teeth should behorizontally oriented or angled 1-2°. The number of grooves (4) in eachanode (1) are balanced with the number of teeth (15) in each anode,which again is a function of size and current density. The currentdensity on the active anode surface facing the cathode can vary between0.3-1.5 A/cm². Two or more anodes form an anode “cluster” (FIG. 1) whichare connected to anode raisers (6) and via the anode beam to the busbarsystem, in a similar way as for a prebake anode obvious to a personskilled in the art of aluminium production. This makes it easy andeconomically favourable to retrofit existing prebake and Søderberg cellsto use present invented anodes. In addition, these new anodes are easyto adjust and replace whenever necessary.

The shape of the anode teeth (15) and grooves have been modelled andoptimised in a reference system, in which the physical parameters likeviscosity, bubble size, etc. are optimised to fit the cryolite-oxygensystem an an Hall-Heroult cell with inert anodes. The model shows thatgas is released by drainage from all the sides of the anode, protectingthe anode from reacting with dissolved aluminium, but most of the gas isreleased from the end of the grooves which also set up the main streamin the inter polar room and between the anodes.

The anode can also be shaped in such a way that the bottom of the anodefacing the cathode can be shaped like a cone or a roof with 3 or moreplanes with angled or straight surfaces facing upwards towards a hole(16) where produced anode gas easily and efficiently can be transportedaway from the active anode surface and escape, and at the same time setup a circulation pattern around the anode (see FIG. 5). The electrolytein the anode hole (16) will be turbulent and well suited for aluminaaddition (11). The gas-induced bath circulation will make sure thatadded alumina efficiently is distributed around the anode keeping thealumina concentration around the anode constant at a predeterminedlevel. Further, the anode is shaped to set up a circulation pattern thatdistributes fresh electrolyte to all parts of the cell.

The anode to cathode distance can be kept at a minimum because of thesmall oxygen bubbles (10) produced at the anodes (1) efficiently areremoved from the inter polar room via the grooves and the sides of theanodes. To keep the produced heat within the cell, there can beinsulation on top (9) of the anode and in the bottom of the cathode potlining (7). The anode top is covered by a lid (14).The top of the anodeshould be insulated to run the cell thermally in balance with a reducedinter polar distance compared to traditional Hall-Hèroult cells

The buoyancy-generated bubble forces (gas-lift effect) on one side andthe flow resistance on the other hand to give a net motion of theelectrolyte (FIG. 3) to provide the required alumina dissolution andsupply, as well as separation of the products. This is accomplished byforming the exterior of the anode (13) in a way that is optimised withrespect to flow behaviour (see FIG. 3), and the direction of the flow isset up by sloping the bottom of the grooves in the desired direction(ex. FIG. 4). The direction of the sloped grooves can be changed fromone anode to the other, and even on the same anode, to set up desiredflow patterns and loops in the cell (FIG. 4). The anodes shouldpreferably be totally immersed to give a strong and controlledelectrolyte circulation.

The cell is located in a steel container, or in a container made ofanother suitable material. The container has a thermal insulating lining(7) and a refractory lining with excellent resistance to chemicalcorrosion by both fluoride-based electrolyte and produced aluminium (2).Alumina is preferably fed more or less continuously, or in very smallbatches (semi-continuously), through one or more feeding points (11) andinto the highly turbulent flow region of the electrolyte between theelectrodes of the cell (FIG. 2). This will allow a fast and reliabledissolution of alumina, even at low bath temperatures and/or highcryolite ratios of the electrolyte without muck formation at the bottomof the cell. These anodes are connected to a peripheral busbar systemvia connectors (6), in which the temperatures can be controlled througha cooling system, if necessary. In particular, the anode and/or theanode connections can be cooled by water cooling or other liquidcoolants, by gas cooling, or by use of heat pipes.

The off-gases and evaporated electrolyte formed in the cell during theelectrolysis process will be collected in the top part (14) of the cellabove the anodes. The off-gases can then be extracted from the cellthrough an exhaust system. The exhaust system can be coupled to thealumina feeding system (11) of the cell, and the hot off-gasses can beutilised for preheating of the alumina feed stock. Optionally, thefinely dispersed alumina particles in the feed stock may act as a gascleaning system, in which the off-gasses are completely and/or partiallystripped from any electrolyte droplets, particles, dust and/or fluoridepollutants in the off-gasses from the cell. The cleaned exhaust gas fromthe cell is then connected to the gas collector system of the potline.The particulates of alumina which are fed to the cell should be as fineas possible.

The present cell design achieves controlled drainage of produced gas anda well defined flow pattern in the electrolysis cell, which are ofcrucial importance to obtain a rapid alumina dissolution anddistribution at a constant and high concentration. By keeping the widthof the anode teeth/bars low (FIG. 4) and with only 1-5° slope towardsthe grooves perpendicular to the anode teeth one obtains a uniformcurrent distribution on the anode teeth and low bubble layer inducedvoltage drop. To avoid localised area of high current densities at theanodes, all the corners (including the corners and edges of the anodeand the grooves) are smoothened/rounded to give a uniform flowcharacteristic and current density. Hence, the unfortunate consequenceof previously patented design solutions is avoided, where clusters ofanode “flower pots”, “bolts” or “rods” are positioned horizontally orwith tilted bottom will give turbulence around the anode “rods” and lesstendency for distribution of alumina to the periphery of the cell, whichrequires a higher number of alumina feeders in the cell to obtain auniformly high alumina concentration. The chance of alumina build ups atthe bottom of the cell (muck formation) is also considered to be lesslikely with the present invention.

A reduction in the exposed cathodic surface area will reduce thecontamination levels of anode material in the produced metal, thusreducing the anodic corrosion during the electrolysis process, which isdifficult to obtain in a retrofit cell unless a complete new cell isdesigned. However, a reduction in the anodic corrosion can be obtainedby reducing the anodic current density (for example by increasing anodicsurface area) and by lowering the operating and/or anode temperature.

The shown multi-monopolar anode clusters (1) may obviously bemanufactured as several smaller units and assembled to form an anode ofthe desired dimensions. In addition all the inert anode clusters (1) canbe exchanged whenever necessary.

Continuous operation of the said electrolysis cell requires the use ofdimensionally stable inert anodes (1). The anodes are preferably made ofmetals, ceramic materials, metal ceramic composites (cermets) orcombinations thereof, with high electrical conductivity. The cathodes(2) can be non-wetted carbon-based or wettable by aluminium in order tooperate the cell at constant interpolar distances (5) Wettable cathodesare preferentially made from a mixture of carbon and titanium diboride,zirconium diboride or mixtures thereof, or by adhering layer(s) ofaluminiumwettable materials to traditional carbon blocks. Likewise, thecathode can also be made of carbon-based cathode blocks, or from carboncomposites of other electrically conducting refractory hard metals (RHM)based on borides, carbides, nitrides or suicides, or combinations and/orcomposites thereof. The electrical connections to the anodes arepreferentially inserted through the lid (14) as shown in FIG. 1. Theconnections (8) to the cathodes (collector bars) are inserted throughthe cathode potlining (7) well known to a person skilled in the art.

The invented cell can be operated at a low interpolar distance (5) tosave energy during aluminium electrowinning. Low interpolar distancesimplies that the heat generated in the electrolyte can be reducedcompared to traditional Hall-Hèroult cells. The magnetic field of thecell and the busbar system have to be optimised to make operation with avery low inter electrode distance feasible without the risk of shortcircuiting the electrodes, which will destroy the anode material andreduce current efficiency. The energy balance of the cell can hence beregulated by designing a correct thermal insulation (7) in the sides andthe bottom is necessary, as well as in the cell top (9, 14). The cellcan then be operated with a self-regulating frozen ledge covering theside walls well known to a person skilled in the art. The anode shouldpreferably be totally immersed in the electrolyte to achieve sufficientelectrolyte flow and thermal balance in the cell.

Excess heat generated must be withdrawn from the cell through the sidesof the cell. The cell liner (7) is preferably made of densely sinteredrefractory materials with excellent corrosion resistance toward the usedelectrolyte and aluminium. Suggested materials in addition to carbonbased cathode blocks are silicon carbide, silicon nitride, aluminiumnitride, and combinations thereof or composites thereof. Additionally,at least parts of the cell lining can be protected from oxidising orreducing conditions by utilising protective layers of materials thatdiffers from the bulk of the dense cell liner described above. Suchprotective layers can be made of oxide materials, for instance aluminiumoxide or materials consisting of a compound of one or several of theoxide components of the anode material and additionally one or moreoxide components.

The invented cell is designed for operation at temperatures ranging from880° C. to 970° C., and preferably in the range 900-940° C. The lowelectrolyte temperatures are attainable by use of an electrolyte basedon sodium fluoride and aluminium fluoride, possibly in combination withalkaline and alkaline earth halides. The composition of the electrolyteis chosen to yield (relatively) high alumina solubility, low liquidustemperature and a suitable density to enhance the separation of gas,metal and electrolyte.

To reduce the dissolution of the anode material, it is beneficial tokeep the temperature at the anode surface (interface) as low as possiblewithout the risk of freeze out since the saturation limits of the anodematerials are reduced with falling temperature. By designing the anodeassemble in such a way that there is a net flux of heat from the bathinto the active surface of the anode, a few degrees lower anode surfacecan be obtained. The anode and/or the anode connections can be cooled toprovide heat exchange heat recovery and/or temperature control of theanode and/or the cathode, The anode and/or the anode connections can becooled by water cooling or other liquid coolants by gas cooling, or byuse of heat pipes. In addition, one can introduce an internal coolingcircuit in the anode using for example a heat-pipe. U.S. Pat. No.4,737,247 shows an example of how a heat-pipe can be used for otherapplications than cooling the anode.

The accumulation of gas underneath the anode causes an extra voltagedrop. The gas volume as well as the resistance are strongly dependent onthe size of the gas bubbles and the size of the active anode, i.e. thedistance the produced anode gas bubbles have to travel to escape fromthe edges of the lower anode surface. Oxygen bubbles produced on inertanodes in cryolite are extremely small (1-2 mm) compared to CO₂ oncarbon anodes. The effect is more accumulated oxygen gas volume underthe inert anodes compared to CO₂, and it limits the optimum size of theinert anode. The active anode surface therefore has to be shaped toefficiently drain away the produced gas from this surface. In thepresent invention the surface of the active anode parts (called “teeth”)is V-shaped leading the gas to the grooves, and the width of the teethmust be minimized according to acceptable bubble layer resistance andcurrent distribution induced by accumulation of gas on these anodeteeth. This aspect of inert anode technology is discussed by Solheim andThonstad, without the authors stating the optimum dimensions.

It should be understood that the suggested aluminium electrowinning cellas presented in the example relating to FIGS. 1-5, represents only oneparticular embodiment of the cell, which may be used to perform themethod of electrolysis according to the invention.

1. A method for electrolytic production of aluminum metal from anelectrolyte including aluminum oxide, the method comprising: performingelectrolysis in an electrowinning cell comprising at least one inertanode and at least one cathode, the at least one anode and the at leastone cathode being arranged so as to face each other, wherein the atleast one anode evolves oxygen gas and aluminum is discharged onto theat least one cathode during the electrolysis, the at least one cathodebeing substantially horizontal; and directing the oxygen gas to flowinto grooves in an electroactive surface of the at least one anode so asto be drained away from an interpolar room, and so as to establish andenforce an electrolyte flow pattern between the at least one cathode andthe at least one anode and over the at least one anode, wherein thegrooves of the at least one anode define a plurality of anode teeth,each of the anode teeth having a V-shaped bottom surface which slopesfrom a center line of a respective anode tooth toward an adjacentgroove, and wherein the grooves are sloped in a longitudinal directionof the grooves and away from the at least one cathode.
 2. A method inaccordance with claim 1, wherein at least one of the at least one anodeand an anode connection is configured to be cooled so as to provide atleast one of (1) heat exchange with at least one of the at least oneanode and the at least one cathode, (2) heat recovery from at least oneof the at least one anode and the at least one cathode, and (3)temperature control.
 3. A method in accordance with claim 1, wherein atleast one of the at least one anode and an anode connection isconfigured to be cooled by means of liquid cooling, gas cooling, or bythe use of heat pipes.
 4. A method in accordance with claim 1, whereinfeeding of alumina to the cell is continuous or semi-continuous, andwherein the alumina fed to the cell contains as fine particulates aspossible.
 5. A method in accordance with claim 1, wherein the cell usesan electrolyte with a temperature in the range of 880-970° C.
 6. Anelectrowinning cell for electrolytic production of aluminum metal froman electrolyte including aluminum oxide, the cell comprising: at leastone inert anode and at least one cathode, the at least one anode and theat least one anode being arranged so as to face each other, the at leastone cathode being substantially horizontal, wherein the anode isconfigured to evolve oxygen gas during an electrolysis process in whichaluminum is discharged onto the at least one cathode such that theoxygen gas enforces an electrolyte flow pattern, wherein the electrolyteflow pattern is to be established between the at least one cathode andthe at least one anode and over the at least one anode; and groovesarranged in an electroactive surface of the at least one anode so as todrain away oxygen from an interpolar room, wherein the grooves of the atleast one anode define a plurality of anode teeth, each of the anodeteeth having a V-shaped bottom surface which slopes from a center lineof a respective anode tooth toward an adjacent groove, and wherein thegrooves are sloped in a longitudinal direction of the grooves and awayfrom the at least one cathode.
 7. An electrowinning cell in accordancewith claim 6, wherein the grooves have a depth of 1-3 cm and a width of1-3 cm.
 8. An electrowinning cell in accordance with claim 7, whereinthe bottom surface of each of the anode teeth is sloped 1-5° from thecenter line of the respective anode tooth towards an adjacent groove soas to efficiently drain produced gas into the adjacent groove.
 9. Anelectrowinning cell in accordance with claim 7, wherein the bottomsurface of each of the anode teeth is sloped 1-2° from the center lineof the respective anode tooth towards an adjacent groove, and whereinthe grooves are sloped in the longitudinal direction of the grooves andaway from the at least one cathode at an angle of 1-5° so as to obtainefficient drainage of produced gas collected in the grooves andestablish a desired flow pattern in the electrolyte.
 10. Anelectrowinning cell in accordance with claim 7, wherein each of theanode teeth has a width of 10-20 cm so as to obtain a uniform currentdensity and a low bubble layer resistance.
 11. An electrowinning cell inaccordance with claim 6, wherein corners and edges of the grooves and atleast one anode are at least one of smoothened and rounded so as toprovide a uniform flow characteristic and current density.
 12. Anelectrowinning cell in accordance with claim 6, wherein a top surface ofthe at least one anode is shaped to set up a circulation pattern fordistributing fresh electrolyte to all parts of the cell.
 13. Anelectrowinning cell in accordance with claim 6, wherein a top of the atleast one anode is insulated above a bath level around stubs as well asa cathode bottom to make it possible to run the cell thermally inbalance with a reduced inter polar distance as compared to traditionalHall-Heroult cells.
 14. An electrowinning cell in accordance with claim6, wherein the at least one anode is totally immersed in the electrolyteso as to achieve sufficient electrolyte flow and thermal balance in thecell.
 15. An electrowinning cell in accordance with claim 6, wherein theat least one anode comprises a plurality of anodes, and wherein two ormore anodes form an anode cluster, the anode cluster being connected toan anode raiser, and being connected to a busbar system via an anodebeam.
 16. An electrowinning cell in accordance with claim 15, whereinthe plurality of anodes comprises a plurality of anode clusters, andwherein the anode clusters are arranged so as to orient the grooves insuch a way that produced oxygen in the grooves sets up an electrolyticflow pattern that facilitates sufficient electrolytic flow velocity toobtain uniform distribution of alumina in the cell without muckformation.
 17. An electrowinning cell in accordance with claim 16,wherein the anode clusters are arranged at an optimized position withrespect to an orientation of the grooves and an orientation of side andcenter channels so as to provide a desired alumina mixing anddistribution.
 18. An electrowinning cell in accordance with claim 15,wherein the plurality of anodes comprises a plurality of anode clusters,and wherein the anode clusters are arranged at an optimized positionwith respect to an orientation of the grooves and an orientation of sideand center channels so as to provide a desired alumina mixing anddistribution.
 19. An electrowinning cell in accordance with claim 6,wherein the bottom surface of each of the at least one anode iscone-shaped or roof-shaped with three or more planes which includesurfaces angled towards a hole in a top surface of the grooves whereproduced gas can escape.
 20. An electrowinning cell in accordance withclaim 6, wherein the at least one anode has a ceramic outer surface, andwherein a center portion of the at least one anode is made of anelectrical conducting material including a cermet or a metal or acombination thereof.
 21. An electrowinning cell in accordance with claim6, wherein the at least one anode is comprised of a plurality of smallerunits integrated in one larger unit.
 22. An electrowinning cell inaccordance with claim 6, wherein the cell is connected to at least onegas exhaust system for extracting and collecting gasses from anelectrolysis chamber.
 23. An electrowinning cell in accordance withclaim 6, further comprising an exhaust system which is connected to analumina feeding system in which hot off-gasses are used for at least oneof heating alumina feed stock and cleaning of the off-gasses from thecell to remove at least one of fluoride vapors, fluoride particulatesand dust before entering a gas collection system.
 24. An electrowinningcell in accordance with claim 6, wherein the at least one cathode ismanufactured from carbon blocks or carbon covered or mixed with anelectrically conductive refractory hard material (RHM) based on borides,carbides, nitrides, silicides or mixtures thereof.
 25. An electrowinningcell in accordance with claim 6, wherein the at least one cathode ismade of horizontal carbon blocks or drained carbon composite blocks. 26.An electrowinning cell in accordance with claim 6, wherein the at leastone cathode comprises an aluminum pool, the aluminum pool beingstabilized by an optimized busbar system magnetic field.
 27. Anelectrowinning cell in accordance with claim 6, wherein the cell has asidewall lining made of an electrically non-conductive material.
 28. Anelectrowinning cell in accordance with claim 6, wherein a sidewalllining of the cell is made of a material selected from aluminum oxide,aluminum nitride, silicon carbide, silicon nitride, and combinationsthereof or composites thereof.
 29. An electrowinning cell in accordancewith claim 6, wherein the at least one anode comprises a plurality ofanodes, and wherein the cell further comprises at least one feedingpoint for alumina located at a position close to high-turbulence areasin the electrolyte, and in an area between two or more of the anodes.